Successful growth in greenhouses is strongly dependent on an appropriate amount of light penetration to the plants so that the photosynthesis generates the desired development of the plants. During recent years shadow curtains have been used, the primary purpose of which is to prevent undesirable sunshine so that the plants do not overheat. Of all sunshine about 45% is visible light, that is photosynthetic light, while the remainder, about 55%, only gives heat. An ideal shadow curtain should reduce the photosynthetic light minimally and reflect all other light maximally.
The plant's requirements for photosynthetic light vary considerably from one type of plant to another and certain plants are so called short day plants which require short days to bloom quickly. If the natural day is not sufficiently short the plant must be fooled, which is achieved with blackout curtains which completely shut out the light. However blackout can cause the plant to stretch itself in an undesirable way or cause blooming to be delayed. One disadvantage with the shadow curtain is that it blacks out the greenhouse to such an extent that at the same time it is impossible to work inside. The blackout curtain can also have the characteristics of an energy curtain, that is, it should reduct the flow of heat to the cold greenhouse and thereby reduce water evaporation from the earth and plants and it should reflect heat radiation from the heating pipes, plants, earth, etc. in the greenhouse.
As different plants are variously sensitive to light and temperature very stringent requirements are placed on the growers' experience and knowhow that the correct steps are taken at the right time.